1. Field of the Invention
The present invention relates to the field of semiconductor integrated circuit manufacturing, and more specifically, to a phase-shifting mask and a process for fabricating a phase-shifting mask.
2. Discussion of Related Art
Improvements in photolithography have allowed higher density and faster speed to be attained in semiconductor Integrated Circuits (ICs) by continually shrinking the devices. According to the Rayleigh criterion, the minimum Critical Dimension (CD) which can be resolved by a wafer stepper is directly proportional to the wavelength of the illumination source and inversely proportional to the Numerical Aperture (NA) of the projection lens. However, diffraction will degrade the aerial image when the CD becomes smaller than the actinic wavelength. The actinic wavelength is the wavelength of light at which a mask is used in a wafer stepper to selectively expose photoresist on a substrate.
Photolithography in the sub-actinic wavelength regime will benefit from wavefront engineering with Resolution Enhancement Techniques (RETs), such as Phase-Shifting Masks (PSMs), to achieve a sufficiently wide process latitude. Unlike a conventional binary mask that only modulates the amplitude of light, a PSM further controls the phase of light to take advantage of destructive interference to mitigate the detrimental effects of diffraction. An Alternating PSM (AltPSM) is particularly useful for patterning very small CDs, such as the gate length of a transistor in a device. AltPSM improves contrast by introducing a phase shift of 180 degrees between the light transmitted through adjacent clear openings to force the amplitude between the two images to zero.
A phase shift of 180 degrees can be implemented creating a difference in the optical path lengths through adjacent openings in an opaque layer, such as chrome. An additive process may be used to deposit a transparent layer, such as Spin-On-Glass (SOG), through openings in the chrome onto a transparent substrate, such as fused silica or quartz, followed by removal of the transparent layer in alternate openings. However, an additive process is susceptible to optical mismatch of materials in the light path and accompanying internal losses at the interfaces. Thus, it is more common to use a subtractive process to etch a trench into the quartz substrate in alternate openings.
However, an AltPSM that is fabricated with a subtractive process will scatter incident light off the sidewalls and bottom corners of the etched openings. The waveguide effect causes an aerial image imbalance which is manifested as a CD error and a placement error. The CD of the etched opening becomes smaller than the CD of the unetched opening. The space between the two openings appears displaced from the unetched opening towards the etched opening.
The aerial image in an AltPSM can be balanced in several ways. A CD biasing approach enlarges the etched opening relative to the unetched opening to balance the aerial image. As shown in FIG. 1, the phase-shifted opening 101 has a trench in the quartz 107 with a depth 117 and a width 111. The non-phase-shifted opening 103 has no trench and a width 113. The phase-shifted opening 101 and the non-phase-shifted opening 103 are separated by an opaque layer 105 with a width 115. The depth 117 corresponds to a phase shift of 180 degrees between the phase-shifted opening 101 and the non-phase-shifted opening 103. The width 111 of the trench includes a bias 109 per edge to increase transmission of the phase-shifted opening 101 relative to the non-phase-shifted opening 103.
An etchback approach, such as with an isotropic wet etch, recesses the sidewalls and corners laterally under the chrome to balance the aerial image. The etchback approach may be one-sided or two-sided. As shown in FIG. 2, the one-sided version of the etchback approach undercuts the substrate below the edges of the opaque layer 205 around the phase-shifted opening 201 only. The undercut 209 per edge increases transmission of the phase-shifted opening 201 relative to the non-phase-shifted opening 203 which is not undercut. The trench in the phase-shifted opening 201 has a depth 219 before the etchback and a depth 217 after the etchback. The depth 217 corresponds to a phase shift of 180 degrees.
The etchback approach may also be two-sided. As shown in FIG. 3, the two-sided version of the etchback approach undercuts the substrate below the edges of the opaque layer 305 around both the phase-shifted opening 301 and the non-phase-shifted opening 303. The phase-shifted opening 301 has a depth 119 before the etchback and a depth 317 after the etchback. The non-phase-shifted opening 303 only has a depth 339 after the etchback. A depth difference 341 is maintained between the two trenches which corresponds to a phase shift of 180 degrees. The undercut 309 per edge of the phase-shifted opening 301 and the undercut 329 per edge of the non-phase-shifted opening 303 determines the quartz width 525 underneath the chrome between the phase-shifted opening 301 and the non-phase-shifted opening 303.
As shown in FIG. 4, a dual-trench approach can also balance the aerial image by etching a deep trench with a depth 417 in the phase-shifted opening 401 and an adjacent shallow trench with a depth 439 in the non-phase-shifted opening 403. The two trenches are separated by chrome with a width 415. A depth difference 441 is maintained between the two trenches which corresponds to a phase shift of 180 degrees.
The various approaches for balancing the aerial image have disadvantages. The CD biasing approach is constrained to the discrete values available on the design grid. The etchback approach may cause defects, such as chipping or delamination of the overhanging chrome between adjacent openings. The dual-trench approach adds complexity and cost by requiring additional processing.
Thus, what is needed is a phase-shifting mask with balanced transmission and phase and a process for fabricating such a phase-shifting mask.